1. Field of the Invention.
The present invention relates to semiconductor devices whose current flow is controlled by layers which are oxidized over part of their areas, and more particularly to layers which have been modified in order to control the extent and shape of the oxidized regions, and most particularly to devices, especially lasers and vertical cavity surface emitting lasers (VCSELs), which utilize such conductive elements. The present invention furthermore relates to the formation of VCSELs which emit at visible and infrared wavelengths which reside on non-GaAs substrates, and VCSELs whose emission wavelengths are precisely controlled.
2. Description of the Prior Art
Vertical-cavity surface-emitting lasers (VCSELs) whose current flow is controlled by lateral oxidation processes show the best performances of any VCSELs in terms of low threshold current and high efficiency. In oxidized VCSELs the oxidation occurs in the lateral direction from the sides of etched mesas in the VCSEL wafers, typically under the conditions of 425xc2x0 C. temperature with high water-vapor content. Presently however, the lateral oxidation process is controlled only through careful control of the timing, temperature, and the sizes of the mesas. This presents difficulties in the manufacurability of such VCSELs, because the current apertures may not be the same from wafer to wafer, or even within a single wafer. Furthermore, since there is no definite stopping mechanism for the oxidation process other than removal from the oxidation environment, the reliability of oxidized VCSELs has not been very high. VCSELs or any other light emitting devices employing laterally oxidized layers have been strictly limited only to structures which have been grown upon gallium arsenide (GaAs) substrates and emit light at wavelengths limited to the region bounded by 0.63 xcexcm and 1.1 xcexcm. Since VCSELs are presently the subject of intense research and development, a great deal of results and advancements are published monthly. are presently the subject of intense research and development, a great deal of results and advancements are published monthly.
Most reports of the oxidation process describe oxidation in layers of aluminum arsenide (AlAs) or aluminum gallium arsenide (AlxGa1xe2x88x92xAs) where the Al concentration, x, is close to unity. As reported by Choquette, et al. in xe2x80x9cLow threshold Voltage Vertical-Cavity Lasers Fabricated by Selective Oxidation,xe2x80x9d which appeared in Electronics Letters, volume 24, pp. 2043-2044, 1994, reducing the Al concentration from x=1.0 to x=0.96 reduces the oxidation rate by more than one order of magnitude. At x=0.87, the oxidation rate is reduced by two orders of magnitude compared to x=1.0. Due to the extreme sensitivity of the oxidation rate to the Al concentration and the fact that Al concentration may vary from wafer to wafer or even over the area of a single wafer, the manufacturability of oxidized VCSELs has been questioned. In the very recent publication by Choquette et al., entitled xe2x80x9cFabrication and Performance of Selectively Oxidized Vertical-Cavity Lasers,xe2x80x9d which appeared in IEEE Photonics Technology Letters, vol. 7, pp. 1237-1239, (November 1995), this problem was noted followed by the observation that xe2x80x9cTherefore, stringent compositional control may be necessary for wafer scale manufacture of uniformly sized oxide apertures.xe2x80x9d
A limited form of lateral control of oxidation is reported in the publication by Dallesasse, et al. entitled xe2x80x9cHydrolyzation Oxidation of AlxGa1xe2x88x92xAsxe2x80x94AlAsxe2x80x94GaAs Quantum Well Heterostructures and Superlattices,xe2x80x9d which appeared in Applied Physics Letters, volume 57, pp. 2844-2846, 1990. The same work is also described in U.S. Pat. Nos. 5,262,360 and 5,373,522, both by Holonyak and Dallesasse. In that work, GaAsxe2x80x94AlAs superlattices were interdiffused in selected regions by impurity-induced layer disordering (IILD). The interdiffusion was essentially complete in the selected regions, thus the interdiffused regions comprised an AlGaAs compound having an Al concentration being approximately uniform and equal to the average Al concentration of the original constituent AlAs and GaAs layers. The oxidation proceeded through the superlattice regions but not significantly into the interdiffused regions. The superlattice was not doped and contained no other structure from which to fabricate any electronic or optoelectronic device. No attempt was made to form any kind of conductive aperture or boundary.
Implantation enhanced interdiffusion (IEI) is another method for interdiffusing thin semiconductor layers and is described by Cibert et al. in the publication entitled xe2x80x9cKinetics of Implantation Enhanced Interdiffusion of Ga and Al at GaAsxe2x80x94AlxGa1xe2x88x92xAs Interfaces,xe2x80x9d which appeared in Applied Physics Letters, volume 49, pp. 223-225, 1986.
Due to the much lower refractive index of aluminum oxide compared to AlAs (about 1.6 compared to 3.0) oxidation of an AlAs layer within a VCSEL cavity shifts the cavity resonance to a shorter wavelength as reported by Choquette et al. in xe2x80x9cCavity Characteristics of Selectively Oxidized Vertical-Cavity Lasers,xe2x80x9d which appeared in Applied Physics Letters, volume 66, pp. 3413-3415, in 1995.
Formation of VCSELs which emit a wavelengths longer than about 11 xcexc m has been difficult in the prior art. Despite numerous efforts toward developing 1.3-1.55 xcexcm emitting VCSELs, only recently as room-temperature continuous-wave emission been reported as in the publication by Babic et al. entitled xe2x80x9cRoom-Temperature Continuous-Wave Operation of 1.54-xcexcm Vertical-Cavity Lasers,xe2x80x9d which appeared in IEEE Photonics Technology Letters, vol. 7, pp. 1225-1227 (November, 1995). In that work, fabrication was accomplished by fusing semiconductor mirrors and active regions epitaxially grown on three separate substrates. Another approach to forming 1.3-1.55 xcexcm emitting VCSELs is to grow semiconductor mirrors of aluminum arsenide antimonide (AlAsSb) and aluminum gallium arsenide antimonide (AlGaAsSb) on indium phosphide (InP) substrates as reported by Blum et al., in the publication entitled xe2x80x9cElectrical and Optical Characteristics of AlAsSb/GaAsSb Distributed Bragg Reflectors for Surface Emitting Lasers,xe2x80x9d which appeared in Applied Physics Letters, vol. 67, pp. 3233-3235 (November 1995).
It is therefore an object of the invention to provide a partially oxidized electrically conductive element in which the lateral extent of the oxidation is controlled.
It is another object of the invention to provide an oxidized VCSEL which is manufacturable.
It is yet another object of the invention to provide an oxidized VCSEL which is reliable.
It is yet another object of the invention to provide an oxidized VCSEL whose emission wavelength is precisely controlled on a fine scale.
It is yet another object of the invention to provide an oxidized VCSEL which emits light at a wavelength greater than 1.2 xcexcm.
According to one broad aspect of the invention, there is provided a conductive element which is substantially conducting in one region and which is oxidized and therefore substantially nonconducting in another region, the conducting region having been made resistive to oxidation compared to the nonconducting region.
According to another broad aspect of the invention, there is provided a VCSEL whose current flow is constrained by a conductive aperture surrounded by oxidized material having predetermined lateral dimensions comprising: a substrate, a first mirror situated above the substrate, a first conductive spacer situated above the first mirror and below the light emitting material, a second conductive spacer situated above the light emitting material; a conductive element comprising an oxidizing layer which has been oxidized in a first non-conducting region and which has been modified to resist oxidation in a second, conductive region; a second mirror situated above the second conductive spacer, a first contact for electrically contacting to the conducting element, and a second contact for electrically contacting a material of a second conductive type, the first and second mirrors and all material between forming an optical cavity having a cavity resonance at a nominal wavelength, and means for injecting electrical current through the conducting element and into the light emitting material, thereby causing the VCSEL to emit a beam of light at or near nominal wavelength.
According to another broad aspect of the invention, the emission wavelengths of such VCSELs are controlled by controlling aperture diameters of the conductive elements and the total thickness of oxidizing layer or layers.
According to another broad aspect of the invention, VCSELs whose emission wavelengths are longer than 1.2 xcexcm are formed by oxidizing at least portions of the first (bottom) mirror or by forming a conductive aperture with a controlled oxidation process.
Other objects and features of the present invention will be apparent from the following detailed description of the preferred embodiments.